54 THE PARENT STOCK OF THE INSECTA. 



those branchise whicli were situated near the centre of 

 the body, neither too much in front nor too far back, 

 would serve the most eificiently as propellers. The same 

 causes which have determined the position of the legs 

 would affect also the wings. Thus a division of labour 

 would be effected, the branchise on the posterior seg- 

 ments of the thorax would be devoted to locomotion, 

 those of the abdomen to respiration. This would tend 

 to increase the development of the thoracic segments, 

 already somewhat enlarged to receive the muscles of the 

 legs. 



That wings may be of use to the insects under water 

 is proved by the very interesting case of Polynema 

 natans, which I discovered in 1862,^ and which uses its 

 wings to swim with. This, however, is a rare case, 

 and it would seem that the principal use of the wings 

 was primordially to enable the mature forms to pass 

 from pond to pond, thus securing fresh habitats, and 

 perhaps avoiding in-and-in breeding. If so, the 

 development of wings would tend to be relegated to a 

 late period of life ; and by the tendency to the inherit- 

 ance of characters of corresponding ages, to which Mr. 

 Darwin has called attention,^ the development of wmgs 

 would be associated with the maturity of the insect. 

 Thus the late acquisition of wings in the insects generally, 

 seems to be itself an indication of their descent from a 

 stock which was at one period aquatic in its habits, and 

 which probably resembled the present larvae of Chloeon 

 in form, but had thoracic as well as abdominal 

 branchia3. 



If these views are correct, the genus Camjpodea must 

 be regarded as a form of remarkable interest, since it 

 is the living representative of a prima3val tyjDC, from 

 which not only the Collembola and Thysanura, but 

 the other great orders of insects, have all derived their 

 origin. 



^ ' Linnean Transactions,' 1863. 



^ ' Origin of Species,' 4tli cd., pp. 14 and 97. 



